-LRB- CNN -RRB- -- It is widely agreed that Wednesday night 's presidential debate is crucial for Mitt Romney : This is almost assuredly his last chance to turn around the election . What is less obvious is that the stakes are sky-high for Barack Obama , too : This debate could have a powerful impact upon his ability to govern in a second term .

Romney is dangerously close to blowing a campaign that many election models said he should easily win . Yes , three national polls published Monday -- including CNN -- showed the two candidates within three points of each other . But for Romney , the problem is that as the polls go up and down each week , there is one constant : As Real Clear Politics demonstrates in its averaging process , Obama stays ahead and has been for nearly a year . The news for the GOP from battleground states is even worse , especially in the Midwest .

Unless he wants to depend on a wing and a prayer , Romney is thus in the unenviable position that he has to shake up the dynamics of the race on Wednesday night . Translated , that means he has to score a convincing victory in the eyes of voters . Simply being as good as Obama wo n't cut it -- a tie goes to the leader .

How to watch , clip and share the debate

Can Romney pull off a debate victory ? Of course -- umm , theoretically . As he proved in the GOP debates , he is quick on his feet , can throw a punch as well as counterpunch , and he looks presidential -- all assets on television . But his campaign has left him with so much ground to make up that it will be very hard : In the space of 90 minutes , he has to convince voters for the first time that he has a better plan for the economy , make the case that Obama does n't and prove much more likable than ever before . And all the while , he will have to fend off jabs from Obama , who can draw from a rich trove of Romney mistakes in the past . So , yes , Romney can still win , but it will be darn hard . No wonder he has been practicing so much .

It would appear , then , that Obama can simply go for caution , choosing a clinch in the center of the ring over hard punches , and walking away with a tie . But on closer examination , Obama ought to be pressing for a victory , too .

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In some polls over recent weeks , especially from key states , the president has now opened up a second possible path to re-election . For a long time , his campaign advisers have assumed that he would win but that his margin of victory would be narrow -- less than three points . Even now , his advisers -- even as they are quietly confident about the ultimate outcome -- are running scared , assuming the race will likely close significantly in the final weeks .

But it is becoming apparent there is another possibility : Contrary to much conventional wisdom , Obama may actually be able to bust open this race , sweeping almost every state he won four years ago and rolling up a victory margin of perhaps five points or more .

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The difference between a big win and a small win for Obama is , to draw upon a famous Mark Twain phrase , almost the difference between lightning and a lightning bug .

Looking beyond November , what really matters to Obama is whether he can effectively govern in a second term . If he wins big , he can persuasively argue that the American people have spoken loudly and clearly , choosing his path into the future over Romney 's . At minimum , he will claim a mandate in favor of higher taxes on the affluent , a strong safety net for those in trouble and a cautious approach toward reducing Medicare , Medicaid and Social Security costs . Republicans will have a much harder time challenging his mandate if he wins big .

Moreover , a big Obama victory would keep the Senate safely in Democratic hands and -- less likely -- might put a few House seats in play . Obama campaign advisers have been building a powerful ground game for months ; they have far more offices in battleground states than Romney . Their hope is that if they turn out enough voters , a rising tide will lift lots of boats downstream . Especially in the Senate , they entertain hopes that a Democratic candidate can come within three points of Obama . If he were to break beyond 53 % in key states , that could elect a lot of Democrats .

Add together the possibility of a convincing mandate with congressional results that may be far better than expected a few months ago and what do you have : bingo , a resurgent Obama heading toward a second term . To some of his advisers , that would make it far more likely he could achieve a `` grand bargain '' on deficits , breaking open the deadlock now paralyzing Washington and holding back corporate investment .

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In short , Obama has a great deal riding on Wednesday 's debate , too . Members of his team may talk as if they have less at stake than their opponent , but in their hearts , they know that if Obama can put Romney away in their first debate , he could well have some lightning at his command in a second term .

The opinions expressed in this commentary are solely those of David Gergen .

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David Gergen : Debate may represent Romney 's last big chance to turn around the race

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He says the stakes are also sky-high for Obama , even if he wins election

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Obama needs a big victory to claim broad support for his plans for second term , Gergen says